Bioshock InfiniteDO NOT BUY ON STEAM! It is cheaper by $6 at greenmangaming with voucher code: GMG20-F202F-UI40F
This is one of those games everyone praises, but will probably only last you one playthrough or so. I'd recommend torrenting or waiting until it's $5, like the rest of the series. Do not buy.

Don't Starve
An interesting survival game that will challenge you and will occupy you when you need a quick lower-end game to play. I'd recommend picking this up. Buy it.

Endless Space
Looks to be a space empire simulator. I have not played it, but if it interests you, 66% off is a good price point to jump in at. Buy it if you're interested.

Hotline Miami
I beat this game in 5 hours and didn't really enjoy it. It's a top-down game that is quick and brutal, emphasizing one-shot kills and strategy. At $2.49, it's kind of hard to pass up, considering it's considered to be a good indie game by others. Buy it.

Toki Tori 2+
This was in a Humble Bundle for $1 and I still didn't buy it. Why would you buy it at $9+?

Left 4 Dead 2
Buy it. It's an arcade horror/survival game that encourages teamwork and discourages Rambo-esque gameplay. Includes the campaigns of the first game and has a lot of replay value. Buy buy buy buy.

Scribblenauts Unlimited
A game where you create things to solve puzzles. It's quite comical. Cheapest it's been, and it's worth it. Buy it if you're interested.